WO2015027287A1

WO2015027287A1 - Customised spacers to assess pre-planned alignment and stability and to assist with component alignment in total knee arthroplasty

Info

Publication number: WO2015027287A1
Application number: PCT/AU2014/000866
Authority: WO
Inventors: Scott Fletcher
Original assignee: The Australian On-Line Prosthetic Company
Priority date: 2013-09-02
Filing date: 2014-09-02
Publication date: 2015-03-05
Also published as: WO2015027288A1

Abstract

A customised patient specific spacer insertable between a resected tibia and a distal femur prior to resection of a distal femur, the spacer enabling checking patient soft tissue balance prior to resection of the distal and/or posterior femur to ensure required soft tissue balance after resection of the femur. The spacer comprises a body having an underside surface and an opposite upper surface and at least one region of a first thickness measured, from the underside surface to the upper surface. The upper surface opposes a part of a distal femur when in situ, the spacer when placed between a tibial plate and a distal femur prior to resection al lowing determination of required soft tissue balance prior to resection of the femur.

Description

Customised Spacers

to assess pre-planned alignment and stability and to assist with component alignment in Total Knee Arthroplasty

BACKGROUND

[01] The present invention relates to surgical equipment and more particularly to a new piece of surgical equipment comprising a customised spacer used during total knee arthroplasty. The present invention also relates to a method of use of the spacer to enable more accurate placement of knee joint arthroplasty components. The spacer is used to assess alignment of knee joint and assessment of ligament stability and balance prior to definitive femoral resection by a surgeon. Th invention also relates to a spacer whic separately allows placement of femora] and tibial components in knee arthroplasty.

PRIOR ART

[02] Knee arthroplasty is a welt -known surgical procedure by which a diseased and/or damaged natural knee joint is replaced by a prosthetic knee joint. Typical knee prostheses include a tibial component, a femoral component, and a patellar component.

[03] Various knee prostheses are disclosed in the prio art. Examples are disclosed in US Patent 5,593,44 to Robertson Jr. US patent 5.782,925 to Colleran United States Patent 5,800,552 the contents of which are incorporated by reference herein. Examples of resurfacing types of total knee prosthetic devices are also disclosed in the following US patents also incorporated by reference herein. U.S. Pat.No.3, 774,244 to Walker; US patent No.3,728,742 to Avertll et al. U.S. Pat No.4,081 ,866 and U.S. Pat. No. 4,207,627 to Cloutier. Modern total knee replacement involves the resurfacing of the femoral condyles with a metallic component, roughly approximating the shape of the anatomical femoral condyles, and resurfacing the tibial plateau with usually, but not exclusively, a polyethylene component, having a metallic tibial base plate. Ideally the femoral component should be congruent with the top of the tibial component in order to minimise wear of a surface liner which is usually polyethylene. During normal movements of the knee, rotation of the femur upon the tibia occurs with roll back of the femoral condyles upon the tibia, particularly when the knee is flexed.

[04] If the plan of the tibial plat when fitted to the tibia is misaligned with the resected proximal surface of the tibia, uneven wear will result between the articular surfaces. A patient may not notice the misalignment and uneven loading of the femoral component on the tibial component but where the loading is concentrated through one condyle wear is accelerated. This may lead to a reduction of up to 50% of the normal, life of the prosthesis

[05] The femoral component generally includes a pair of spaced apart condylar portions, the surfaces of which articulate with a portion of the polyethylene tibial component. In known total knee prostheses the articular surface of the distal femur and proximal tibia are usually but not exclusively replaced with respective metal and plastic condylar-type articular bearing components. The knee prostheses provide adequate rotational and translational freedom and require minimal bone resection to accommodate the components within the boundaries of the available joint space. The patella-femoral joint may also be resurfaced by a third prosthetic component, as well. The femoral, tibial and patella prosthetic resurfacing components are affixed to respective, surgically prepared adjacent bone structure by cementing or by biological bone ingrowth. The femoral component is usually but not exclusively a metallic allo construction such as cobalt- chrome alloy and provides medial and lateral condylar bearing surfaces of similar shape and geometry as the natural distal femur. The tibial component can be made entirely of ultra high molecular weight polyethylene or can be comprised of a metallic base and stem component distaJly and an interlocking plastic (UHMWPE) component, proximally. The plastic tibial plateau bearing surfaces are often of concave multi-radius geometry to more or less match the articular geometry of the mating femoral condyles, depending upon the desired design mechanics of primary femoro -tibial

7 motion, e.g. the flexion-extension, including posterior rollback and rotational and translational articular motions.

[06 ] The femoral and tibial components are positioned on the respective side of the knee joint and are not mechanicall connected or linked together. The components are intended to be disposed such that it will allow more accurate simulation of anatomical geometry or dynamic action at an implant site in a patient.

[07] Additionally, in resurfacing types of total knee prostheses the tibial plateau bearing surface geometry can assume a variety of configurations, depending upon the desired extent of articular contact and associated translational (medial-lateral and anterior-posterior) and rotational (axial and varus-valgus) secondary femoro-tibial motions. These various secondary motions allow the resurfaced knee to function in a natural-like biomeehanical manner in conjunction with the surroundin ligamentous and muscle structures about the knee joint. The viable soft tissue structures functionally maintain the femoral and tibial bearing surfaces in contact, provide the necessary levels of constraining force to achieve knee joint stability, and decelerate the principal motion in flexion-extension and secondary motions, such as axial rotation, etc, in a controlled manner,

[08] The objective in knee replacements is to simulate with a dynamic implant, natural knee function as closely as possible. Any improvement which allows a surgeon greater capacity in achieving this objective is desirable. The articulation of the femoral condyles with the tibial plateau bearing surfaces involves complex biomechanics allowing primary femoro- tibial flexion and extension, and secondary motions of axial and varus- valgus rotations and anterior-posterior and medial-lateral translations, all of which occur in the normal knee joint. The knee joint reaction forces during primary or secondary motion are principally supported by the tibial bearing surfaces, and are transferred to the underlying fixation interfaces and adjacent supportive bone structures, In a normal knee, physiological femoro- tibial rollback starts at the onset of knee flexion and is generally mostl completed by 40 degrees of flexion. This rollback is accompanied by a transitional motion of rolling and sliding. In the normal knee, these complex interactions are accompanied by complex active interaction of the anterior and posterior cruciate ligaments and other surrounding adjacent soft tissue structures.

[09] The above is a description of known biomechanics of a knee joint prosthesis. As knee prostheses attempt to simulate as closely as possible the patient's biomechanics this required simulation necessitates extremely accurate fixation so any means which enables surgeon to meet this objective is desirable.

[1.0] Accurate placement of components in total knee arthroplasty has become increasingly identified as an issue that affects both function of the knee replacement and how long the knee replacement is likely to survive. Methods to help the surgeon place components accurately include conventional methods, computer navigation and patient specific resection guides. Navigation, by default, has provided an intra-operative method to validate the planned resections. To date, there has been no objective method of validating the planned resections when using Patient Matched Guides (PMG). On the basis that the patient matched guides may be inaccurate or alternatively, that the guides may be inaccurately placed on the bone, it is desirable to have a method for validating the intra-operative resectio positions on the tibia and femur, prior to the surgeon performing the actual resection.

[1 1] Proper rotation of th femoral component in total knee arthroplasty (T A) is critical, as improper rotation can lead to various adverse consequences including instabilities such as but not limited to patella/femoral instability, anterior knee pain, arthrofibrosis, and flexion instability. Variou methods are available for determining accurate femoral component rotation. One method is a measured resection technique favoured by many surgeons in which bone landmarks (femoral epicondyles, posterior femoral condyles, or the ntero posterior axis) are the primary references for determination of femoral component rotation. Another method used IS w ga balancing methodology in which the femoral component is positioned parallel to the resected proximal tibia with each collateral ligament equall tensioned. Each method used has some attendant disadvantages and relies on the surgeon to intr operative]}* accurately confirm angles and distances using bone landmarks (measured resection technique) or correct soft tissue tensioning in the circumstance where gap-balancing methodology is used to determine femoral component rotation in a total knee arthroplasty (T A),

[12] The use of a measured resection technique for determination of femoral component rotation relies on accurate intraoperative identification of numerous bone landmarks. Advocates of this technique recommend placement of the femoral component either parallel to the transepicondylar axis, perpendicular to the anteroposterio axis, or approximately 3° to 4° externally rotated relative to the posterior condylar axis. Use of each of these landmarks is associated with pitfalls that risk improper rotation of the femoral component.

[13] The transepicondylar axis has been recognized as an acceptable axis to guide femoral implant rotation.. This is supported by kinematic analyses that have demonstrated better coronal plane stability (tower incidence and magnitude of femoral condylar lift-off) if the femoral component is placed parallel to the transepicondylar axis. There are instances where accurate surgeon identification of the transepicondylar axis is not frequently accomplished, which can result in flexion gap asymmetry.

[14] Accuracy of epicondylar identification can be assessed post operatively to determine if there had been accurate epicondylar identification. One study performed showed that when 74 T As were assessed in which the femoral epicondyles were marked with pins mtraoperativeiy, and postoperative CT scans were performed to assess the accuracy, it w¾s observed that the epicondyles were correctly identified to within ±3° in only 75% of the cases, with a wide range of error from 6° of external rotation to 1 1° of internal rotation. There are significant errors in, intraoperative surgeon identification of the femoral epicondyles. The error can be as much as 28° - 1 1 ° external rotation to 17° of internal rotation . Studies show that the ability of the surgeon to accurately and reproducibiy identity the transepicond lar axis land mark which is a good landmark to determine femoral component rotatton,, can be quite poor. Many TKA instrument systems have been deveioped that reference the posterior condylar axis to assist the snrgeon in performing femoral bone resections that result in femoral component placement 3° to 4° externally rotated to this axis.

[15] Although simple to use in the operating room, there are problems with using the posterior condylar" axis to reference femoral component rotation. Investigations have shown wide anatomic variations in the relationship of the posterior condylar axis to the transepicondylar axis 1°-10°. Therefore, if a patient's anatomical relationship of the posterior condylar axis is 7° of external rotation vs the transepicondylar axis and the instnimentation used places the femoral component in 3° of external rotation, the femoral component will be internally rotated 4° relative to the transepicondylar axis. Hypoplasia or erosion of the posterior aspect of the lateral femoral condyle in knees with valgus deformity will lead to erroneous femoral component position if the posterior condylar axis is used as the primary determinant of femoral component rotation,

[16] The anteroposterior axis, traversing from the deepest point of the trochlear groove to the center of the intercondylar notch, is an additional hone landmark used to determine femoral component rotation. Another method is to place the coronal plane position of the femoral component perpendicular to the anteroposterior axis and observed enhancement of both stability as well as patellar tracking. There can be wide range of external rotatio error when using the anteroposterior axis as a determinant of femoral component rotation.

[17] Another technique currently used with femoral component rotation is a gap balancing methodology, i which the femoral component is positioned parallel to the resected proximal tibia with each collateral ligament equally tensioned. Comparisons have been made in outcomes between use of gap balancing techniques and measured resectio techniques using bone landmarks as the primary references to achieve femoral component rotation. There are studies which identified rotational errors of at least 3° occurring in 45% of patients when rotation was determined from fixed bony landmarks. Anatomic bony landmarks (measured resection) are used in determining rotation of the femoral component. Gap balancing can also be used.

[18] Wide variations in femoral component position have been observed with use of both the posterior condylar axis and anteroposterior axis. When the posterior condylar axis was used, the femoral component was positioned at a mean of 0.4° internally rotated as compared with gap balancing (range, 1 · internal to 13° of external rotation). Gap-balancing methods when used demonstrate a significantly lower incidence and magnitude of femoral condylar lift-off. This indicates the superiority of the use of the gap- balancing technique in obtaining a balanced flexion gap. Statistically, the gap balancing technique provides a more accurate and reproducible way to obtain satisfactory flexion gap stability.

[19] There are currently variations on two main options when assessing axial alignment of the distal femur, (femoral implant rotation)

[20] Ligament balance Option: After coronal plane soft tissue release (to correct pre-operative varus or valgus deformity), the surgeon uses tension within remaining (post - release) soft tissue envelope to determine femoral implant rotation. Thi may be assessed by the traditional LCS method or with newer tensiometers place in the flexion gap. Navigation systems may be used to assess this ligament balance option. The advantage of this option is that it reliably gives approximate equal tensions on the medial and lateral sides of the joint.

[21] Possible disadvantages: Released (after soft tissue correction of deformity) may allow for a rotatory mal-alignment of the femoral component, leading to internal rotational deformities following varus correction and external, rotational deformities following valgus correction.

[22] The soft tissue flexion space balance is often assessed / performed with the extensor mechanism dislocated. This reflection of the extensor mechanism may give rise to a balanced flexion gap with the extensor mechanism dislocated, but may give mal-assessment of soft tissue tension when the patello-femoral mechanism is enlocated. This ma lead to internal femoral mal-rotational of the femoral component.

[23] Measured Resection Option: Most surgeons who use Measured Resection, resect the proximal tibia at 0 degrees (coronal plane) to the mechanical axis of the tibia, and externally rotate the femoral component to the PCA, the epicondylar axis, to the AP axis of some combination of this triad. The advantages are that most of the time, the outcome is a reasonably balanced knee.The disadvantage is that there may be a measured resection mismatch between the changes made to the proximal tibial coronal plane correction and the measured resection external rotational change of femur, that leads to a flexion space mismatch in soft tissue balance with consequent ligamentous instabilities, (e.g. mid flexion instabilities).

[24] Although there are multiple methods which currently may be used to determine correct femoral component rotation durin TKA, surgeons still experience difficulty in accuratel determining correct femoral rotation and also in precise identification of critical bone landmarks when deciding correct femoral component rotation using a measured resection methodology.

INVENTION

[25] The present inventio provides a customised spacer used during total knee artliroplasty which enables more accurate placement of knee joint arthroplast components and to enable assessment of alignment of knee joint and assessment of ligament stabilit and balance prior to definitive femoral resection by a surgeon and to simulate in advance of resection soft tissue balance as occurs after resection. The spacer is customised for each individual patient and allows placement of femoral and tibia! components in knee arthroplasty.

[26] The spacer accommodates the flexion and extension gaps created durin knee arthroplasty after resection of bone from one side of the joint. A created space or gap invention requires the creation of spaces that occupy In use. a customised patient, specific spacer is placed in the gap created after excision of the upper tibia and before definitive resection of the femoral bone. Preferably the customised spacer is created as a customised patient specific device which is prepared after assessment of pre-operative alignment.

[27] In practice, the customised spacer may be manufactured as part of the creation of a patient specific guide prepared for accurate joint resection or alternatively, may be purpose built during the operation. The spacer, when in situ, allows a determination of accuracy of tibial resection, prior to planned femoral resection, by specific reference to other bone landmarks, specifically, but not exclusively, the medial malleolus of tibia.

[2S] The spacer in situ allows a determination or assessment of the alignment and balance of the knee should the femoral resection be performed as pre- operatively planned. Further, the use of the spacer allows for placement of pins within the adjacent uncut bone (usually femoral) to facilitate or enable the accurate resection of the adjacent (femoral) bone, according to preoperative plans.

[29 ] In its broadest form the present invention comprises:

a spacer for use in knee surgery and which enables validation of soft tissue balancing before resection to simulate soft tissue balance of a knee joint after resection, the spacer comprising a body having a base surface which, when in situ conforms to an opposing surface of a tibial implant, an upper surface which oppose at least one femoral surface, the body having a region of at least one thickness measured from the base surface to a location on said upper surface wherein the spacer is sized to approximate the area defined by femoral condyles when viewed axially.

[30] According to a preferred embodiment the base surface is generally planar and the spacer has a region of first thickness measured between, the base surface and upper surface in a region which when the spacer is in situ, aligns generally with a medial region of a distal femur. The spacer also has a region of second thickness measured between the base surface an u er surface in a region which when the spacer is in situ, aligns generally with a lateral region of a distal femur. The spacer has an intermediate region of a third thickness measured between the base surface and upper surface in a region which when the spacer is in situ, aligns generally with a region intermediate the lateral and medial region of a distal femur. Preferably, the upper surface of the spacer at a; medial region conforms generally to the shape of a medial femoral condyle and the upper surface of the spacer at a lateral region conforms generally to the shape of a lateral femoral condyle. The thickness of the spacer ma vary in different locations on the spacer. For instance the spacer at a medial region may be thinner that the thickness of the spacer at the lateral region. The thickness at the medial and lateral regions may be the same of different. referably the medial region of the space will normally be thinner that that at the lateral region.

[31] The spacer may be symmetric or asymmetric about a vertical axis. The thicknesses of the first and second thickness regions are calculated as the difference between the thickness of a tibial plate and a planned thickness o respective femoral resection. The spacer preferably has a thickness falling within the range of 8mm - 20mm and is preferably manufactured fi"^om either a plastics or metallic material. Plastics may be selected from materials including polyethylene or polyurethane or other hard suitable compound plastics.

[32] In another broad form the present invention comprises:

a customised patient specific spacer msertable between a resected tibia and a distal femur prior to resection of a distal femur, the spacer enabling checking patient soft tissue balance prior to resection of tire distal and/or posterior femur to ensure required soft tissue balance after resection of the femur, the spacer comprising body having an underside surface and an opposite upper surface and at least one region of a First thickness measured from the underside surface to the upper surface, the upper surface opposing a part of a distal femur when in situ, the spacer when placed between a tibial plate and a distal femur prior to resection allowing determination of required soft tissue balance pri or to resection of the femur [33] I;Iyb;rid Femoral

The method utilises the advantages and eliminates the disadvantages of the ligament Balance and the Measured Resection options described previously .herein. The plan for alignment k assisted, pre-operatively using a Hybrid Knee web site. A known amount of bone to be resected from a proximal tibia and a distal femur is planned and the femoral and tibial instruments to be used during the knee operation are manufactured. The spacers are also manufactured.

[34] The tibial PSI is used to assist resection level and resection alignment for the proximal tibia. The spacer is inset ted between resected tibia and non resected femur to assess alignment and stability in both flexio ( extensor mechanism enlocated) and extension. Secondary release of ligaments may be per.formed.The distal femur and the AP resection of femur (and femoral rotation) are all governed by the Spacers.

[35] in Hybrid Femoral component alignment with Spacers, the planned femoral rotation is mathematically correlated to the planned coronal plane tibial re-alignment. (Hybrid alignment algorithm). For example, if the preoperative coronal plane alignment is 3 degrees of varus, and the operative resection is to 0 degrees of varus (ie neutral to the mechanical axis in the coronal plane), then the femur is externally rotated 3 degrees. This means that there is a mathematical correlation between the change in coronal plane of tibia and the change in (external) rotation of the implant on distal femur . The primary driver of change to femoral rotation is not ligamentous balance between tibia and femur, but the calculated change in bone (eoronal plane tibia) alignment.

[36] The Spacers encourage the native femur to rotate to the planned femoral rotational alignment. An excessive ligament release will not result i a secondary rotator implant mal-alignment . This avoids the disadvantages of the prior art such as tire l igament distraction methods whereb an over released soft tissue envelope may allow excessive distraction between tibia and femur and consequent ma!-rotation of the femoral implant.

I I [37] The present invention provides an alternative to the known prior art and the shortcomings identified. The foregoing and other objects and advantages will appear from the description to follow. In the description reference is made to the accompanying representations, which forms a part hereof, and in which is shown by way of illustration specific embodiments in which the invention ma be practiced. These embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilised and that structural changes may be made without departin from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[38] The present invention will now be described wit reference to preferred but non limiting embodiments and with reference to the accompanying illustrations wherein:

Figure la: shows a table of preoperative planned resections from a

proximal tibia, posterior femur and distal femur.

Figure lb; shows coronal view of a knee joint highlighting planned tibial and femoral bone resections and prior to bone resection.

Figures 2a. shows the extension gap i the coronal plane;

Figure 2 b shows the flexion gap in sagittal plane.

Figure 3a shows a coronal view of a customised spacer.

Figure 4a shows a coronal plane view in extension

Figure 4t> shows a sagittal plane view on flexion, Figure 4c shows how the position of the extension and flexion spacers can be validated with reference to known bone landmark (e.g. media! malleolus)

Figure 5, shows a coronal view of knee with two pins placed in distal femur. These pins will facilitate distal femora! resection.

Figures 6(i) ~<v)

shows a series of figures in sagittal profile that demonstrates the placement of a jig that interfaces with the custom spacer (i), insertion of AP pins (ii), and removal of jig with pins remaining in situ.

Figure 7a &b:

shows the AP and chamfer resection of distal femur enabled by placement of two pins that pass from distal (femur) to proximal (femur) and the placement of these pins gui ded by a jig that interfaces wi th a flexion spacer .

Figure 8 shows a perspective view of a spacer according to one non limiting embodiment.

DETAILED DESCRIPTION

[39] Referring to Figure la there is shown a table of preoperative planned resections from a proximal tibia, posterior femur and distal femur. This example of a pre-operative plan indicates how much bone is to be removed from the bone ends. Figure 1 refers to an the example where the preoperative plan requires the removal of 7mm from the medial and lateral sides of the tibia, 8mm from the posterior medial condyle and 5mm from the lateral posterior condyle. 9mm is to be removed from the medial distal condyle and 5mm from the lateral distal condyle.

[40] In Figure lb there is shown a coronal view of a knee joint 1 comprising a distal femur 2 and proximal tibia 3. Tibia 3 is highlighted at region 4 to show a planned resection thickness of 7mm at the medial location

5 and lateral location 6. which highlighting planned tibial and femoral bone resections and prior to bone resection. Figure 2a shows in the coronal plane the knee of figure 1 with corresponding numbering and an extensio gap 7 between femur 2 and tibia 3. Figure 2b shows the flexion gap 7 in a sagi ttal plane.

[41] Figure 3a shows according to one embodiment a coronal view of a customised spacer 10. There will a flexion spacer and an extension spacer. The thickness of the spacer in any one compartment (media, lateral, flexion and extension) will be determined by the following formula:

Spacer thickness = prosthesis thickness (18mm) - the thickness of femoral condyle that is planned to be removed. Spacer 10 comprises a body 1 1 having a generally planar underside surface 12 which opposes a surface of a resected tibial plateau. Spacer 10 also has an uppe surface 13 which has contours which oppose distal femoral condyles. According to die embodiment shown, spacer 10 includes on a medial side 14 a surface 15 which when the spacer is in situ, opposes a medial femoral condyle. On a lateral side, upper surface 16 opposes lateral femoral condyle. Intermediate surface 15 arid 16 is a profile 17 whic assists with the location of spacer 10 between the femoral condyles. Spacer J 0 further comprise openings 18, 1 , 20, 21 and 22 which can be used for locating pins, locating jigs and the like to assist with alignments and measurements during surgery.

[42 ] Figure 4a shows with reference to the joint of figure 2 a coronal plane view of a spacer 20 with similar characteristics to the spacer 10 of figure 3 when the knee is. in extension. These figures show the custom spacers in situ and before distal and posterior femoral resection. Figure 4a shows a coronal plane view in extension and Figure 4b shows a sagittal plane view of the space 20 when the femur is in flexion. That alignment and soft tissue balance in both extension and flexion can be assessed at this stage using the spacer 20. The thickness of customised spacer 20 is equal to 18mm minus the planned thickness of femoral resection. This allows a determination of how much bone from the femoral side is required to make up the 18mm. Figu re 4c shows how the position of the extension and flexion spacer 20 can be validated with reference to known bone landmark on a tibiafe.g. medial malleolus) [43] Figure 5 shows the joint of figure lb wit correspondin numbering setting a. resection, of distal femur 2, Shown is spacer 20 and a corona! view of two pins 25, 26 placed in distal femur and which will facilitate distal femoral resection. With, the custom extension spacer 20 in situ and with alignment and stability confirmed, resection of distal femur can be enabled by placement of two pins 25, 26 in the distal femur passing from anterior to posterior through a guide that interfaces wit the custom spacer 20.

[44] Figures 6(i) -(v) shows a series of knee joint figures similar to the sagittal view of the knee joint 1 of figure 2b also in sagittal profile that demonstrate tire placement of a jig 30 that interfaces with a custom spacer 31 --see 6(i). Insertion of AP pins 22 is shown in figure 6(ii). Figure 6(iii) shows removal of jig 30 with pins 33 and 34 remaining in situ . The distal resectio guide 30 is shown in Figure 6(iv) and figure 6(v) shows the femur 36 after resection at location 37.

[45] Figure 7a&b shows the AP and chamfer resection of a distal femur 40 enabled by placement of two pins 43 and 42 that pass from distal (femur) 40 to proximal (femur) (i) and the placement of the pins 41 and 42 is guided by a jig 43 that interfaces with and penetrates anterior spaces in the flexion spacer 44 . Figure 7(11 ) shows resected femur 40.

[46] Figure 8 shows a perspective view of a spacer 50 according to one non limiting embodiment. Spacer 50 which enables validation of soft tissue balancing before resection to simulate soft tissue balance of a knee joint after resection, comprises a body 51 having a base surface 52 which, when in situ conforms to an opposing surface of a tibial plateau. An upper surface 53 opposes when in situ, at least one femoral condyle surface. Body 51 has a region of one thickness measured from the base surface 52 to a location 54 on said upper surface 53. Body 51 has a region of a second thickness measured from the base surface 52 to location 55 on said upper surface 53. Intermediate locations 54 and 55 is an apex region 56 which when the spacer is in situ locates between the femoral condyles which respectively oppose locations 54 and 55 in a similar manner that that described with reference to

35 figure 3 b. Spacer 50 further comprises openings 57, 58, 59, 60 and 61 which can be used for locating pins, locating jigs and the like to assist with alignments and measurements during surgery. The locations shown are optiona and it will be appreciated that alternative openings of different sizes and locations may be included as required.

PRINCIPLES OF OPERATION

[47] The method described uses a new customised spacer that aligns the lower limb in a predetermined manner. The spacer allows for intra-operati e assessment of this alignment and the stability of tire knee joint with this predetermined alignment.

[48] When alignment and stability is validated, the customised spacers direct placement of the femoral component with reference to the customised spacers. This means that once a femoral component is placed, the alignment and stability will be identical to that validated with the customised spacers and before femoral resection.

[49] Although one spacer will be specific to a particular patient, a kit of spacers ma be made available allowing the surgeon to select an appropriate sized spacer for that patient. Ideally a spacer is supplied which allows checking of soft tissue balancing for a patient. Thus a spacer or a kit of spacers is provided to enable validation the soft tissue balance of the knee that will be present after all resections have been completed. The spacer is used and soft tissue validation assessment made after the tibial resection but before any femoral bone resection .

[50] According to an. apparatus aspect there is provided a kit of spacers comprising at least one customised extension spacer and at least One customised flexio spacer suitable for a particular patient. The spacers allow the following approach to assessment of soft tissue balance using the spacers

[51 ] If it assumed that after tibial resection, that 1. There is a known thickness of the posterior and distal aspects ot the chosen implant (in this example, this thickness - 8mm); and

2. there will be a 10mm construct placed upon the tibia (usually comprising of tibial base late and polyethylene); and

3. a known amount of bone will be resected from each of the distal femoral condyles and each of the posterior condyles, then It is possible to calculate the thickness of the customised spacer so that soft tissue balance can be assessed prior to femoral resection.

[52] If it can be assumed that the flexion and extension gaps will be 18mm (tibia = 10mm and thickness of femora! implant = 8mm, then from the formula, 18mm minus planned femoral resection equals customised spacer thickness. The flexion and extensio customised spacers are used after tibial resection, and before femoral resection. If the customised spacers demonstrate a balanced knee in flexion and extension prior to femoral resection, then it can be assumed that if the femur is resected according to plan, that there will be a balanced knee in flexion and extension after femoral resection and placement of the femoral and tibial components if the customised spacers do not t t, the surgeon will have the option to consider modification of the planned bone resections or soft tissue release prior to definitive femoral resection.

[53] This spacers thus addresses the problem of intra-operative validation of planned femoral and tibial resectio and its concordance with soft tissue balance before femoral resection. The customised spacers can be applied to all total knee arthroplasty rather titan specifically to knee arthroplasty operations that involve use of PMI. The spacers can also be used in arthroplast of other joints (eg hip or Knee arthroplasty) where there has been a resectio on one side of the joint and where the coaptin side of the joint has a planned depth dimension of resection that has yet to be performed.

[54] It will be recognised by persons skilled in the art that numerous variations and modifications may be made to the invention broadly described herein without departing from the overall spirit and scope of the invention.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

t. A spacer for use in knee surgery and which enables validation of soft tissue balancing before resection to simulate soft tissue balance of a knee joint after resection, the spacer comprising a body having a base surface which, when in situ conforms to an opposing surface of a tibial plateau, an upper surface which opposes at least one femoral surface, the body having a region of at least one thickness measured from the base surface to a location on said upper surface; wherein the spacer is sized to approximate the area defined by femoral condyles when viewed axially.

2. A spacer according to claim 1 wherein the spacer has a region of first thickness measured between the base surface and uppe surface in a region which when the spacer is in situ, aligns generally with a medial region of a distal femur.

3. A spacer according to claim 2 wherein the base is general ly planar,

4. A spacer according to claim 3 wherein the spacer l as a region of second thickness measured between the base surface and upper surface in a region which when the spacer is in situ, aligns generally with a lateral region of a distal femur.

5. A spacer according to claim 4 wherein the spacer has a region of a third thickness measured between the base surface and upper surface in a region which when the spacer is in situ, aligns generally with a region intermediate said lateral and medial region of a distal femur.

6. A spacer according to claim 5 wherein the upper surface of the spacer at a medial region conforms generally to the shape of a medial femoral condyle.

7. A spacer according to claim 6 wherein the upper surface of the spacer at a lateral region conforms generally to the shape of a lateral femoral condyle.

8. A spacer according to claim 7 wherein the spacer at a medial region is thinner that the thickness of the spacer at the lateral region.

9. A spacer according to claim 8 wherein the first and second thicknesses are different.

10. A spacer according to claim 8 wherein the first and second thicknesses are the same.

1 1. A spacer according to claim 10 wherein the spacer is symmetric about a vertical axis

12. A spacer according to claim 1 1 wherein the spacer is asymmetric about a vertical axis

13. A customised spacer according to claim 9 wherein the thickness of the first and second thickness regions are calculated as the difference betwee the thickness of a tibial plate and a planned thickness of respective femoral resection.

14. A customised spacer according to claim wherein the spacer has a thickness falling within the range of 8 mm - 20mm.

15. A customised spacer according to an of the foregoing claims manufactured from a plastics or metallic material.

16. A customised spacer according to claim 15 manufactured from a material selected from polyethylene, polyurethane

17. A customised patient specific spacer insertable between a resected tibia and a distal femur prior to resection of a distal femur, the spacer enabling checking patient soft tissue balance prior to resection of the distal and'or posterior femur to ensure required soft tissue balance after resection of the femur, the spacer comprising a body having an underside surface and an opposite upper surface and at least one region of a first thickness measured from the underside surface to the upper surface, the upper surface opposing a part of a distal femur when in situ, the spacer when placed between a tibial plate and a distal femur prior to resection allowing determination of required soft tissue balance prior to resection of the femur.